Particulate fillers, such as silica, have conventionally been used as reinforcing materials for polymer materials, such as nitrile butadiene rubber (NBR), to improve physical properties such as compressive modulus, tensile strength, abrasion, tear properties and dynamic properties. For example, a silica-filled nitrile butadiene rubber (SFNBR) has been used for ballistic applications.
Because silica has strong filler-filler interactions and adsorbs polar materials, silica-filled rubber compounds exhibit poor dispersion of the filler and poor cure characteristics. Furthermore, conventional SFNBR often may include fibers and are, thus, anisotropic materials having different properties when measured along different directions therein. Currently available SFNBR compositions also have undesirably high specific gravities and compressive modulus. The conventional SFNBR compositions include high contents of NBR, which result in high viscosities (i.e., Mooney viscosities of greater than about 50) and flow issues. Silica has also been shown to significantly retard the cure rate of the SFNBR, which may be attributed to the adsorption of curatives on the silica surface. Furthermore, conventional SFNBR compositions include components that are either difficult to obtain or are not available in the United States.
Recently, there has been considerable interest in forming polymer materials reinforced with nanosized particles, often referred to as “nanocomposites.” Such nanocomposites conventionally include nanoclay particles dispersed in a polymer material. Smectite clays, such as montmorillonite clays, are most commonly used as nanoclays in nanocomposites due to their high cation exchange capacities, large surface area, good surface reactivity and surface adsorptive properties. Depending on an amount of dispersion of the nanoclay particles within the polymer material, the nanocomposite may have either an intercalated structure or an exfoliated structure. In an intercalated nanocomposite, layers of the polymer material are disposed between sheets of the nanoclay particles. In an exfoliated nanocomposite, the polymer material is completely dispersed within the sheets of nanoclay particles.
Development of nanocomposites is difficult, however, because thermodynamic and kinetic barriers inhibit dispersion of the nanoclay particles in the polymer material. For example, the hydrophilic nature of nanoclay particles prevents dispersion and results in formation of aggregates in the polymer material. Furthermore, mixing the nanoclay particles with the polymer material may result in an uncured composition having an undesirably high viscosity and inadequate flow properties for use in certain applications.
Insulative compositions having improved processability and improved properties, as well as methods for making such compositions are desired.